The lithographic process for the manufacture of semiconductor devices, liquid crystal display devices or thin-film magnetic heads, for example, uses projection exposure apparatuses in which an image of a photomask or reticle (hereinafter, “reticle”) is imaged upon a photosensitive substrate through a projection optical system. In such projection exposure apparatuses, in order to assure that the pattern of the reticle is transferred onto the photosensitive substrate at high resolution, the exposure should be carried out while the photosensitive substrate is registered with a best imaging plane (best focus plane) of the projection optical system, within the range of depth of focus. To this end, the position of the best focus plane of the projection optical system, that is, the best focus position, should be detected in accordance with some method. Further, projection optical systems have what can be called an image plane (curvature of field) in which the best focus position differs with the image height.
A known example of methods for measuring the best focus position of a projection optical system is as follows. A single pattern or plural different patterns are illuminated with two illumination lights having principal rays of different tilt angles or two illumination lights having principal rays of the same tilt angle but being incident in symmetrical directions. Then, the spacing of plural pattern images obtained thereby upon a photosensitive substrate is measured or, alternatively, a relative positional deviation between images of the plural patterns being superposed one upon another by moving the photosensitive substrate is measured. The relationship between the defocus amount and the above-described spacing or relative positional deviation is then detected and, on the basis of this relationship, the best focus is calculated. There is another known method in which the best focus position is detected without changing the position of a photosensitive substrate with respect to an optical axis direction of a projection optical system.
The above-described measurement method based on oblique incidence illumination does not need SEM measurement, and it is a method which enables simple and high-throughput and high-precision measurement of best focus position, image plane and astigmatism (astigmatic aberration). Further, the size of a pattern to be used for the above-described positional deviation measurement can be made large as compared with patterns to be used in a method of measuring positions of pattern images having phases changed between zero degrees and ninety degrees for the sake of measurement of best focus, image plane or astigmatism, which is similarly based on positional deviation measurement, or in a method of measuring a positional deviation of an interference pattern produced by interference of two light beams. Therefore, in regard to aerial image measurement, there is an advantage of no necessity of using an enlargement imaging optical system. Additionally, the above-described measurement method based on oblique incidence illumination has a large sensitivity to positional deviation with respect to defocus.
However, it has been found that the relationship between the defocus amount and the amount of positional deviation or the spacing between the pattern images obtained in accordance with the measurement method based on oblique incidence illumination, is not an utter rectilinear relation, in a strict sense. The cause thereof is that, since in the measurement based on oblique incidence illumination there occurs a shift of object spectrum position at a pupil plane, the object spectrum is cut asymmetrically by an aperture stop of a projection lens, such that asymmetrical distortion is created in an aerial image at the imaging plane.
Further, when the asymmetrical object spectrum is denoted by A(f) and the OTF of the projection lens is denoted by O(f), the relation with an amplitude distribution f(x) of an image at the imaging plane is represented by f(x)=F−1[A(f)·O(f)] (F−1 is a Fourier inverse transform). It is seen also from this that wavefront aberration of a projection lens is influential to the aerial image.
A curve 20 in FIG. 7 is a graph wherein a positional deviation amount of a pattern (axis of abscissa) is detected with respect to defocus of a projection lens (axis of ordinate) having spherical aberration produced there. The pattern used for the focus measurement is the pattern of a conventional mark, as shown in FIG. 20. In FIG. 20, denoted at C is the length of the opening of a test pattern TP. This opening has a transmissivity nearly equal to 100%.
It is seen from FIG. 7 that, with the conventional pattern, apparently, the linearity is destroyed.
In the best focus measurement based on oblique incidence illumination in the conventional method described hereinbefore, due to a difference in wavefront aberration of a projection lens of an exposure apparatus or to a difference in wavefront aberration depending on the image height within the angle of view, the relation between the defocus amount and the positional deviation amount differs. Therefore, it is necessary to examine each time the relation between the defocus amount and the positional deviation amount or the spacing.
Further, when the relation between the defocus amount and the positional deviation amount or the spacing is approximated by a linear expression, a value of a defocus amount calculated by using an approximate expression from a positional deviation at a certain imaging plane or a spacing amount contains an error. This is a serious problem for measurement of optical characteristics of a projection optical system, where simplicity and higher precision are required in the future trend of further miniaturization of circuits.
On the other hand, aberrations of a projection lens, such as spherical aberration, image plane (curvature of field), astigmatism (astigmatic aberration), coma (coma aberration), wavefront aberration, and the like, are measured, and it is used in practical evaluation or inspection. Among these aberrations, the waveftont aberration is the aberration of concern. By approximating this wavefront aberration on the basis of a generally used Zernike polynomial, for example, aberrations such as spherical aberration, image plane, astigmatism, coma, and the like, which are factors of the polynomial, can be calculated. The measurement of wavefront aberration is also thus regarded as being important with respect to prediction, by simulation, of process margins of a large variety of device patterns.
U.S. Pat. Nos. 5,828,455 and 5,978,085, for example, propose measuring methods for wavefront aberration. In the measuring methods proposed in these patents, a grid-like pattern is formed on a reticle pattern surface while a pinhole is provided just underneath the center of the grid-like pattern with a small clearance held between them. Further, at the reticle top face, there is a special reticle having a convex lens disposed just above the center of the grid-like pattern. Where this reticle is illuminated by an illumination system of an exposure apparatus, illumination light emitted from the illumination system illuminates the grid pattern placed below it, with an illumination angle (NA) not less than al defined by means of the convex lens. Light passed through the grid pattern passes through the pinhole disposed below it. Here, the light which can pass through the pinhole is limited only to such light as having angles defined by connecting the pinhole and positions of respective points on the grid pattern. Therefore, light beams emitted from respective points on the grid pattern advance as plural lights having different angles.
These lights having different angles impinge at different positions on a pupil plane of the projection lens, and while being influenced by the wavefront aberration of the projection lens, they reach the wafer surface such that respective points of the grid pattern are imaged there. Here, the images of respective points of the grind pattern thus imaged have been affected by different influences of wavefront aberration (phase). More specifically, since light rays advance in the direction of a normal to the wavefront, the imaging position of the image of each point of the grid pattern shifts from an idealistic position by an amount determined by tilt of the corresponding point on the wavefront. In consideration of it, deviations of the images of respective points of the grid pattern from an idealistic grid are measured and, based on it, tilt amounts of the wavefront at respective points in the pupil plane are obtained. Then, by using various mathematical methods, the wavefront aberration is calculated.
The wavefront aberration measuring methods as proposed in the aforementioned U.S. Pat. Nos. 5,828,455 and 5,978,085 are similar to the Hartman method, well known in the art. In the Hartman method, a pinhole is disposed at a pupil plane of a projection lens thereby to restrict the wavefront position and, on the basis of a positional deviation of a pattern image formed by light passed therethrough, the tilt of the wavefront is detected.
In the Hartman method, a pinhole is placed at a pupil plane such that, in regard to the object spectrum, according to equation (1) below, due to the pinhole filter, only information related to a certain small wavefront region can be obtained.
It is desirable to positively control the shape of the object spectrum by disposing a pinhole at a pupil plane (i.e., a pupil filter), as in the Hartman method. However, in practical exposure apparatuses, this is difficult to accomplish because of the space of a barrel or of a purging structure necessary for contamination prevention, for example, and for the reason of cost.
On the other hand, in the methods as proposed in the aforementioned U.S. Pat. Nos. 5,828,455 and 5,978,085, a pinhole is provided just underneath the object surface. Thus, the object spectrum upon the pupil plane corresponds to, unlike equation (1) below, a Fourier transform including a phase term.E(x)=F−1[G(f)·p(f)·w(f)]  (1)                F−1: Fourier inverse transform        E(x): optical amplitude function of image        G(f): object spectrum        w(f): pupil (wavefront) function        p(f): pinhole function        
It is an object of the present invention to provide a reticle having a test pattern by which an optical characteristic of a projection optical system can be measured very precisely, and a method of measuring an optical characteristic of a projection optical system by use of such a reticle.
Also, it is an object of the present invention to provide a reticle having a test pattern by which an optical characteristic of a projection optical system can be measured in accordance with a procedure completely different from that shown in the aforementioned two U.S. patents, and a method of measuring an optical characteristic of a projection optical system by use of such a reticle.
Particularly, it is an object of the present invention to provide a reticle having a test pattern by which at least one of best focus position, astigmatic aberration, curvature of field, and wavefront aberration of a projection optical system can be measured very precisely, and a method of measuring an optical characteristic of a projection optical system by use of such a reticle.